Saturday, August 02, 2014

Dietary gospel on fat is wrong

After decades of health warnings, the idea that steak, cheese and lard are bad for your heart is melting away. The truth is more complex – and delicious

THERE'S a famous scene in Woody Allen's film Sleeper in which two scientists in the year 2173 are discussing the dietary advice of the late 20th century.

"You mean there was no deep fat, no steak or cream pies or hot fudge?" asks one, incredulous. "Those were thought to be unhealthy," replies the other. "Precisely the opposite of what we now know to be true."

We're not quite in Woody Allen territory yet, but steak and cream pies are starting to look a lot less unhealthy than they once did. After 35 years as dietary gospel, the idea that saturated fat is bad for your heart appears to be melting away like a lump of butter in ...

Yet the voices of doubt have been growing for some time. In 2010, scientists pooled the results of 21 studies that had followed 348,000 people for many years. This meta-analysis found "no significant evidence" in support of the idea that saturated fat raises the risk of heart disease (American Journal of Clinical Nutrition, vol 91, p 535).

The doubters were given a further boost by another meta-analysis published in March (Annals of Internal Medicine, vol 160, p 398). It revisited the results of 72 studies involving 640,000 people in 18 countries.

To the surprise of many, it did not find backing for the existing dietary advice. "Current evidence does not clearly support guidelines that encourage high consumption of polyunsaturated fatty acids and low consumption of total saturated fats," it concluded. "Nutritional guidelines... may require reappraisal."

In essence, the study found that people at the extreme ends of the spectrum – that is, those who ate the most or least saturated fat – had the same chance of developing heart disease. High consumption of unsaturated fat seemed to offer no protection. ...

Yet the voices of doubt have been growing for some time. In 2010, scientists pooled the results of 21 studies that had followed 348,000 people for many years. This meta-analysis found "no significant evidence" in support of the idea that saturated fat raises the risk of heart disease (American Journal of Clinical Nutrition, vol 91, p 535).

The doubters were given a further boost by another meta-analysis published in March (Annals of Internal Medicine, vol 160, p 398). It revisited the results of 72 studies involving 640,000 people in 18 countries.

To the surprise of many, it did not find backing for the existing dietary advice. "Current evidence does not clearly support guidelines that encourage high consumption of polyunsaturated fatty acids and low consumption of total saturated fats," it concluded. "Nutritional guidelines... may require reappraisal."

In essence, the study found that people at the extreme ends of the spectrum – that is, those who ate the most or least saturated fat – had the same chance of developing heart disease. High consumption of unsaturated fat seemed to offer no protection.

It’s been more than 40 years since Allen’s movie premiered, but his satire of public health research sadly still resonates. A widely circulated New York Times blog post reported this week on a study purporting to show that people who run at least five minutes a day live around three years longer than those who don’t. This finding was determined to be true after “adjusting for” various characteristics of study subjects—their gender, whether they smoked, any family history of heart disease, and so forth.

The problem with this study—and the many related observational studies on what does or doesn’t make us live longer—is that healthy people are different in all sorts of ways from unhealthy ones. Some of the differences between runners and nonrunners can be accounted for, albeit somewhat imperfectly, by considering observable attributes like height, weight, age, etc. But inevitably there are differences that get left out of the analyses: runners might sleep longer; they might eat more almonds or blueberries; maybe they have less stressful work lives, which in turn facilitate a few minutes of exercise each day; maybe they are less depressed because they have shorter commutes; maybe their commutes involve more walking than driving. The list of other attributes and habits, any of which might contribute to a longer observed life span, extends to infinity. Is it running that accounts for the difference in longevity between runners and nonrunners? Or one of these other, unobserved differences? Who knows?

That's right, it is hard to prove that one diet or lifestyle is healthier than another.

ACCORDING to the Oxford English Dictionary, the expression "heart attack on a plate" was first recorded in 1984 in a newspaper interview with actor Michael Caine. He was living in health-conscious Los Angeles at the time and missing his full English breakfast. That rings true, as it was around then that the US public was being urged to reduce its intake of saturated fat to cut the risk of cardiovascular disease.

Thirty years on, the idea that pigging out on bacon, egg and sausages can lead to a heart attack is second nature to most of us; it is probably the single most influential piece of nutritional advice ever dished out.

But in recent weeks and months a steady drumbeat of media coverage has suggested that saturated fat has been unfairly maligned. "Eat Butter", declared the cover of Time magazine. "Everything we thought we knew about dietary fat is wrong," said the blurb on The Big Fat Surprise: Why butter, meat and cheese belong in a healthy diet, an influential book by investigative journalist Nina Teicholz.

Really? Everything? As usual, the truth is less earth-shattering. Yes, two large reviews of the evidence have cast doubt on the supposedly rock-solid link between saturated fat and cardiovascular disease. Scientific understanding of how the human body handles fat has indeed moved on. And the original research that proved the link has been questioned (see "Heart attack on a plate? The truth about saturated fat"). But it is too soon to declare saturated fat innocent of all charges. Much more research is needed before the nutrition rule book can be rewritten. In any case, meat, butter and cheese already belong in a healthy diet, as long as you don't eat too much of them.

If there is an immediate take-away message, it is that singling out one nutrient at the expense of the wider dietary context is a mistake. In our rush to cut down on saturated fat, we may have inadvertently upped our intake of other unhealthy nutrients, especially sugar. In fact, one of the interesting by-products of the saturated fat debate is that it is helping to reinforce the emerging idea that refined sugar is the real demon in our diets.

The case against sugar is getting stronger, as our story earlier this year spelled out (New Scientist, 1 February, p 34). But it would be a mistake to fixate on sugar at the expense of everything else.